1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a method for forming a conductive film in a semiconductor fabrication process.
2. Discussion of the Background
Generally, a conductive film is formed by a low pressure chemical vapor deposition (LPCVD) process in a vertical furnace or a horizontal furnace. Here, the vertical furnace process will be described.
FIG. 1 is a schematic cross-sectional diagram of an internal structure of a vertical reaction furnace. As shown therein, a vertical furnace 10 is provided with: an outer reaction tube 13; an inner reaction tube 15 of high purity quartz material disposed within a central portion of the outer reaction tube 13; a boat 17 provided in the inner reaction tube 15 and loaded with a plurality of wafers; a heating coil unit 19 in which coils are wound around the outer reaction tube 13; and a plurality of temperature measuring sensors 21 which are disposed at one side of the furnace 10 for measuring the temperature of the outer reaction tube 13. The boat 17 is divided into four sectors 17a, 17b, 17c and 17d. Here, the boat 17 is not physically divided into four sectors, but when depositing a conductive film on each wafer in the vertical furnace 10, the temperature in the outer reaction tube 13 is differently distributed in accordance with each sector of the vertical furnace 10, thereby being divided into four sectors by thermal classification. Each temperature measuring sensor 21 is provided for measuring the temperature of each section and the heating coil unit 19 covers the outer reaction tube 13 for temperature control.
Now, a conventional method for forming a conductive film in the thusly provided furnace will be described referring to FIGS. 2A and 2B.
First, the boat 17 wherein a plurality of wafers are provided is loaded in the inner reaction tube 15 (S0). The next process will be described divided into a decompression step, a deposition step, a purge step and a normal pressure step.
In the decompression step (S1), the pressure in the outer reaction tube 13 is reduced from 760 torr to 90 pascal for 100-110 min., and the temperature thereof is maintained at 570.degree. C.
In the deposition step (S2), after decompressing the outer reaction tube 13, PH.sub.3 and SiH.sub.4 are introduced from a lower part of the outer reaction tube 13 into the inner reaction tube 15 for depositing a conductive film on each wafer. Here, the temperature for the process is maintained at 570.+-.10.degree. C. and the pressure therefor is 950 pascal. Here, the time of the above process may be varied in accordance with the thickness of the conductive film to be deposited on the wafers, ranging between about 20 min. and 2 hours.
In the purge step (S3), in order to externally exhaust toxic gas which has been produced in the deposition step (S2), the outer reaction tube 13 is put under a vacuum by being drained and N.sub.2 is introduced into the outer reaction tube 13, for thereby removing the toxic gas. Here, the pressure in the outer reaction tube 13 ranges between 90 pascal and a vacuum state.
Lastly, in the normal pressure step (S4), after the purge process has been completed, the pressure in the outer reaction tube 13 increases up to atmospheric pressure.
In the thusly provided furnace employing the LPCVD method, each conductive film deposited on the wafer has a different thickness across from the lower section of the reaction tube to the upper section thereof since the deposition gas is introduced into the lower part of the chamber, and thus the gas is more densely distributed in the lower section of the reaction tube, whereas in the upper section thereof the gas is more thinly distributed. Thus, in order to adjust the uniformity of the conductive film deposition, the temperature in the lower section of the reaction tube is lowered by about 10.degree. C. compared to the upper section thereof.
The temperature control is accomplished by the heating coils disposed at an outer side of the chamber and adjusted by each temperature measuring sensor. The temperature control is divided by the four sectors for the convenience of the operation. The temperatures of the four sectors 17a, 17b, 17c and 17d are 570.degree. C., 567.degree. C., 563.degree. C. and 560.degree. C., respectively.
Although there is provided the method for obtaining a conductive film having a uniform thickness, a conductive film in a crystalline condition is deposited on the wafer in sector 17a, and a conductive film in a crystalline and amorphous condition is deposited on the wafer in sectors 17b and 17c, and a conductive film in an amorphous state is deposited on the wafer in sector 17d.
An annealing process is performed after the process for depositing the multi-crystalline silicon, for thus improving the reliability of the deposited film. However, due to the different conditions of the deposited films, the thickness of the amorphous film is shrunk during the annealing process, whereas the shrinkage of the crystalline deposited film is minute, and thus the thicknesses of the deposited films on each wafer of the sectors are formed differently from each other. Accordingly, the conventional method for forming a conductive film has a problem in that the deposited films have differences in properties such as resistance.